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The internal P cycling in coastal sediments during early diagenesis is of great importance to aquatic ecosystems, but its relationship with sulfate reduction is still unclear and lacks visualized evidence. In this study, diffusive gradients in thin films (Zr-oxide DGT and AgI DGT) techniques at a two-dimensional (2D) submillimeter resolution were utilized to synchronously characterize the in situ labile P and dissolved sulfide in coastal sediments around Xiamen Bay, China. The distributions of dissolved sulfide formed from sulfate reduction showed large vertical and horizontal variations, which are mainly controlled by the reactive organic carbon (OC) and redox conditions at different sampling sites. Multiple correlation analysis revealed that significant and linear correlations exist between dissolved sulfide and DGT-labile P, demonstrating that P mobilization is predominantly regulated by sulfide-dependent reductive dissolution of Fe(III)-bound P and sulfate reduction-associated OC mineralization. Both labile P and dissolved sulfide showed lower values at intertidal sites, due to oxic/suboxic conditions favoring weak sulfate reduction and strong P sequestration. Labile P accumulation during flood tides and removal during ebb tides on tidal timescales indicate that the mobilization and detention of P in surface sediment are regulated by tidal cycles and consequential S and Fe redox cycling. The apparent diffusion flux of P implies that the internal P loading can act as a P source to aquatic ecosystems. • High-resolution depth profiles of labile S and P were obtained using the diffusive gradients in thin films technique. • Redox condition and organic carbon contents mainly control sulfate reduction and labile S distributions. • P mobilization is predominantly regulated by sulfate reduction. • Tidal cycle affects mobilization and detention of P in surface sediment. • Internal P loading is an important P source to coastal water. [ABSTRACT FROM AUTHOR]

The release of trace metals caused by industrial effluents and anthropogenic activities has been recorded in the Xixi River estuary, southern China. However, a thorough understanding of the behavior of trace heavy metals in Xixi River sediments is lacking. A total of 12 sediment cores were collected in June and December in the upper estuary section and mouth of the estuary. Here, an in situ high-resolution sampling technique, namely, diffusive gradients in thin films (DGT), was employed to acquire profiles of trace element concentrations and the release of bioavailable metals from sediments in different seasons. A three-step Community Bureau of Reference (BCR) sequential extraction method was used to explore the chemical speciation of trace metals in different seasons and to thereby assess the release potential of trace elements in sediments. The BCR sequential extraction results showed that the trace metals Fe, Mn, Co and Pb were mainly in the residual fraction, which rarely influences living organisms. The total mobile fractions (F1 + F2 + F3) of all trace metals were higher in winter than in summer, suggesting that accumulation occurred from summer to winter. DGT measurements showed that the intensity of sulfate reduction was higher in summer than in winter because of the high temperatures and high organic matter in summer. The intensity of sulfate and Mn(III/IV) reduction increased from the upper estuary section to the lower estuary. Fe(III) reduction decreased in summer but increased slowly in winter. The Pearson correlation results showed that the release of DGT-labile Co in pore water was related to Mn(III/IV) reduction, while the release of DGT-labile Pb was basically not controlled by the Fe-Mn-S redox transition. Abnormally high DGT-labile Pb concentrations were observed at the sampling station (XR3) closest to the estuary in winter, which might have been caused by the high Pb content in the local micro-sediments. • A three-step sequential extraction and DGT were combined to explore the occurrence speciation in sediments. • The release of DGT-labile Co in pore water was related to Mn(III/IV) reduction rather than Fe(III) reduction. • The sampling station (XR3) closest to the estuary has abnormally high concentrations of DGT-labile Pb in winter as a result of the input of external pollution discharge. • Trace metals were mainly in the residual fraction and rarely influenced living organisms. • An accumulation from summer to winter occurred for the total mobile fractions, which are bioavailable for all trace metals and were higher in winter than in summer. [ABSTRACT FROM AUTHOR]